On some architectures, the maximum representable address deviates from
the virtual address range that is accessible by the firmware at boot
time. For instance, on AArch64, UEFI mandates a 4 KB page size, which
limits the address space to 48 bits, while more than that may be
populated on a particular platform, for use by the OS.
So introduce a new macro MAX_ALLOC_ADDRESS, which represent the maximum
address the firmware should take into account when allocating memory
ranges that need to be accessible by the CPU at boot time.